Secretion

SECRETION (secretio) is, on one hand, the release by glands of substances necessary for the vital functions of the organism itself, and on the other, the elimination of metabolic products as unnecessary and harmful ballast. According to this division, it is customary to distinguish secretion in the strict sense (secretory processes) from excretion (excretory processes). Thus, urine and sweat must be classified as excretions, while all other secretions of numerous glands, with the entire diversity of their properties, are classified as secretions. Milk and semen occupy a somewhat special place. On one hand, they cannot be considered waste, harmful to the organism and therefore subject to removal; on the other, they are not necessary for the organism itself; their biological value is determined not in relation to the individual secreting them, but by their task of maintaining the integrity of the species. Glands that release secretions, as well as excretory glands, are equipped with excretory ducts through which the products of their activity are discharged. These glands bear the name of glands of "external secretion," in contrast to glands of "internal secretion"—a whole group of glands not equipped with ducts and releasing their products—"incretions"—into the blood. Some glands, such as the pancreas and the sex glands, combine the functions of external and internal secretion. The mechanism of all glandular secretions is still not entirely clear. The proposed theories of simple physical processes—filtration, diffusion, in which glands were assigned the purely passive role of filtration apparatuses—were abandoned after the works of Ludwig, Claude Bernard, and Heidenhain. Based on the study of glands of external secretion, these researchers provided a number of propositions upon which modern views on secretory processes are built. These propositions are as follows: 1) the secretory power of glands does not depend on blood pressure and, in certain cases (irritation of the secretory nerve), can significantly exceed it; 2) the cells of active and resting glands are morphologically different; 3) the temperature of an active gland is higher than the temperature of the blood bathing it; 4) the consumption of O₂ in an active gland increases; 5) the increase in the gland's production is in direct dependence on the speed of the blood flowing to it; 6) the products that make up glandular secretions are either not found in the blood at all or are present in it in a significantly lower concentration. Based on these propositions, the essence of secretion is reduced to the fact that glandular cells receive raw material from the blood, from which a secretion specific to each gland is synthesized through intracellular processing (secretion in the strict sense), or this material arrives from the same source in a ready-made form (excretory glands). The production of secretion is associated not only with a change in intracellular structure but also with an increase in temperature, which indicates the presence of energy processes occurring in the active gland. The role of excretory glands is more passive and apparently reduces only to the removal of substances already dissolved in the blood, which is why there is no rise in temperature or change in morphological signs during their work. A common phenomenon in the activity of both secretory and excretory glands is an increase in the speed of the flow of blood bathing them. The specific properties of glands, such as the ability of each gland to produce only its own characteristic secretion and to adsorb from the blood the material necessary for that given gland, have not yet found sufficient explanation. Both external and internal secretion are subject to the influence of the nervous system, and it is not possible to isolate any propositions common to all glands (for the innervation of individual glands, see the corresponding articles).